1. Field of the Invention
This invention relates to hydrostatic machines and, more particularly, to a self-sealing hydraulic transfer tube therefor.
2. Description of the Prior Art
Electrical power generating systems in an aircraft often include an integrated drive generator system wherein an aircraft engine is used as a power source for an electrical generating system. In a typical system, a drive shaft connectable to an output shaft driven by the aircraft engine is connected to a mechanical differential, with the differential having an output connected to drive the generator. A variable speed transmission, such as a hydrostatic transmission, is associated with the mechanical differential and is controlled to modify the output of the differential, as required, whereby the input speed to the generator remains constant even though the speed of the drive shaft may vary.
A typical hydrostatic transmission in one form may include a fixed displacement motor hydraulically connected to a coaxial variable displacement pump. The pump and motor may be of the same basic structure, including rotatable cylinders in which pistons reciprocate under the control of a swash plate. The motor and pump are hydraulically connected through a port plate. Specifically, an input shaft of the transmission drives the pump to convert rotary movement to hydraulic pressure, which hydraulic pressure is used for operating the motor to drive an output gear. In order to maintain a supply of oil to the transmission, known systems employ a transfer tube hydraulically coupling a source of oil to the port plate.
A typical transfer tube is installed in hydraulic communication with the port plate by drilling a first or communication hole from an edge to the porting area of the port plate. A second hole, comprising an oil supply hole for receiving the transfer tube, is drilled in the port plate to intersect the communication hole. However, only one portion of the communication hole, i.e. the portion between the oil supply hole and the porting area, is needed. The outer portion of the communication hole must be sealed against oil pressure so that the proper amount of oil is supplied to the hydraulic oil circuit. Known systems have employed a straight recessed Allen socket set screw to seal the outer portion of the communication hole. However, during operation of the integrated drive generating system, this set screw can back out of the hole and cause oil starvation of the hydraulic unit.
Therefore, it is necessary to provide a vibration proof seal in order to prevent oil leakage from the hydrostatic transmission.